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Noise Metrics and Community Response 117 ing 75 dbA of SEL may be computed for a single location for two separate alternative flight routes, and the results compared to identify which alternative provides more noise abatement. Because output information is available in detail, the number of operations may be determined for any combination of noise level, type of operation, and if desired, aircraft type or group. The information provides the public with a level of detail that it often has demanded to assist in understanding of the potential effects of proposed airport development actions. It provides the planner the ability to design noise abatement and land management actions that more pre- cisely address the impacts perceived as meaningful by the public to those identified as significant by regulation. Traditionally, the alternatives are compared through mapping of change in the Numbers of Events Above, a threshold from a baseline condition. Example of Metric Display for Public Information: Examples of Number of Events Above illustrations (160) 7-11 Change in Number-of-Events Above SEL 60 (Annual Average Day) Combined Alternatives (161) 7-12 Example of Number of Events Above Video (prepared for ACRP 02-05 project) 7-13 Time Above The Time Above metric has been available for many years, but has rarely been used as a plan- ning tool for noise abatement or land use mitigation. Output information is computed by the noise model to indicate the amount of time noise exceeds a user-selected threshold. This infor- mation is helpful in evaluations of sleep or speech disturbance when the thresholds are set at lev- els that correlate to the significant effect levels of those activities. One substantial drawback to use of Time Above information is that simultaneous events heard within the same time span are added to the total independently, resulting in overestimates of the total Time Above. This is par- ticularly problematic at threshold levels below 80 dBA near an airport, and at levels below 70 dBA along departure paths. At very low thresholds, the total TA for a day may exceed the number of minutes in a day a result that does not further the analysis of the noise conditions. Time Above results may be presented as either the minutes per day or the percentage of the day that the threshold of interest is exceeded. Example of Metric Display for Public Information: Example of Time Above Video (prepared for ACRP 02-05 project) 7-14 A metric of interest in the evaluation of national parks and other similar properties of signif- icant national interest (monuments, wilderness areas, etc.) is the Time Above Ambient (TAA) noise levels. The calculation of this metric requires knowledge of ambient noise levels at specific locations on the surface, or a digital map of the distribution of non-aircraft ambient noise across the area of interest. The metric is typically applied in areas of very low ambient levels where nat- ural quiet is an essential component of the environment. Consequently, the TAA levels tend to be greatly overestimated by the INM because simultaneous events at low noise levels will be added independently, rather than considered together. Audibility is a metric developed for the National Park Service that is applied in national parks and other park areas where natural quiet is an important part of the visitor experience. The metric is similar to Time Above in that it considers the amount of time each aircraft event is audible. However, audibility considers such exceedances within each of the twenty-four 1/3 octave bands of the frequency spectrum associated with aircraft noise. This measure also is subject to over- estimation through the addition of simultaneous events to a single total.